Note: in British English, flax generally refers to the fibre crop while linseed refers to the oil crop. In American English, the two words are more interchangeable (Muir et al., 2003). In this datasheet, flax will refer to the plant and linseed meal to the oil by-product.

Linseed meal is the by-product of oil production from linseeds (Linum usitatissimum L.). Linseeds are primarily used for the production of linseed oil, which is used in paints and in other industries, such as the manufacture of linoleum. Linseeds and linseed meal have attracted considerable attention since the 1990s due to the presence in the oil of polyunsaturated fatty acids (PUFA), notably alpha-linolenic acid (ALA, an omega-3 fatty acid) and conjugated linoleic acid (CLA). Supplying these fatty acids to the diets of livestock is being used to alter the fatty acid profile of meat, milk and eggs in order to provide health benefits to human consumers. Other benefits include laxative properties and positive effects on the appearance of skin and hair (Newkirk, 2008; Muir et al., 2003). Linseeds and linseed oil contain large amount of lignans, which act in mammalians as phytoestrogens and have anti-carcinogenic properties (Przybylski, 2005).

Processes

Linseeds are rich in oil but their extraction is difficult and often requires double pressing. The seeds are first moisturized to minimize the formation of fine particles and then passed through sets of corrugated and smooth rollers to be cracked and flaked, respectively. When linseeds are processed for food oil production, extraction is done with cold pressing (oil temperature below 35°C), and the resulting oil meal contains significant amounts of residual oil (about 10%). When linseeds are processed for industrial use, the moisturized and flaked seeds are sent to a cooker where they are heated to a temperature of 80-100°C to inactivate enzymes and facilitate the release of oil during pressing. This step also removes toxic substances. The cooked seeds are transferred to the expeller for mechanical extraction, and the resulting cake is fed to the solvent (hexane) extractor. The solvent-extracted cake is desolventized at 100°C and then cooled (Przybylski, 2005).

Linseed meal consists of dark grey pieces (flakes) of variable size with a smooth, slightly curved cut surface (TIS, 2014). Unlike other major meals (soybean, sunflower and rapeseed), the major source of linseed meal available for livestock feeding is the oil-rich expeller type rather than solvent-extracted meal, which is less commonly used (Brunschwig et al., 2010).

Cyanogenic glucosides

Immature linseeds contain glucosidic substances such as linamarin, linustatin, and neolinustatin. At some temperatures (40-50°C), acidity level (pH 2-8), and in the presence of moisture, an enzyme (linase) releases hydrogen cynanide (HCN). In the normal oil extraction process, the high temperature destroys the linase and no HCN is released. However, unprocessed whole seeds and linseed meals processed under a low temperature can be toxic to animals (notably monogastrics), especially if the seed or the cake is wetted before being fed (McDonald et al., 2002). It is recommended to add seeds to boiling water (rather than cold water and bringing them to a boil) at least 5 minutes to rupture the seed coat. This allows the cyanide producing compounds to react and/or be destroyed by the heat, and evaporates any HCN produced. The cooked seeds form into a thick mucilaginous mass (Kohnke et al., 1999). Extraction with trichloroethylene or carbon tetrachloride destroys the glucosides (Przybylski, 2005).

Vitamin B6 antagonist

Linseeds contains linatine, a vitamin B6 (pyridoxine) antagonist, in concentrations ranging from 20 to 100 mg/kg. Symptoms of B6 deficiency can be observed in broilers fed linseeds, and it is recommended that diets containing them or linseed meal be supplemented with vitamin B6 (Newkirk, 2008).

Mucilage

Linseeds contain 2-7% of water-soluble carbohydrates, known as mucilage (mucins), which interfere with both processing and digestion (Przybylski, 2005). Mucilage absorbs water, increasing intestinal viscosity and causing a laxative effect. While this effect can be beneficial in humans, it can impair performance of livestock, notably in young birds (Alzueta et al., 2003). Adding fibre-degrading enzymes and gradually introducing linseeds have been suggested to alleviate these antinutritional effects (Slominski et al., 2006).

Ruminants

Linseed meal is used as a protein source in ruminants. It contains a substantial proportion of rumen-undegradable protein (Dixon et al., 2003a; Frydrych, 1992). Due to its a moderate protein degradability (55%), its metabolizable protein value (about 20% DM, Sauvant et al., 2004) linseed meal is competitive with rapeseed meal as a protein supplement in animal feeding. However, its amino acid composition is less valuable for ruminants than that of rapeseed meal, as it provides 20% less digestible methionine and lysine (Mustafa et al., 2000). The PUFA composition of linseed meal can be used in order to enrich the quality of milk (Brunschwig et al., 2010) and meat products (Normand et al., 2005). It must be noted that linseed meal has a good reputation in ruminant feeding that its proximate analysis does not fully justify. It is possible that the ability of mucilage to absorb water results in an increase in the bulk of the meal, and thus in longer retention times and more thorough microbial digestion (McDonald et al., 2002).

Dairy cattle

Linseed meal is palatable to dairy cows (McDonald et al., 2002). Both expeller and solvent-extracted linseed meals can be used up to 10% of DM as protein supplements in diets containing barley or maize-based concentrates. Solvent-extracted linseed meal has no deleterious effect on digestion of other components of the diet, and parameters of rumen fermentation are similar to those from rapeseed meal, supporting its use as substitute for rapeseed or soybean meal in diets for dairy cattle (Khorasani et al., 1994). The presence of PUFA is beneficial for the health quality of the milk, but it softens milk fat, making it more susceptible to oxidative rancidity (McDonald et al., 2002).

Buffaloes

In India, supplementing a wheat straw-based buffalo diet with up to 1.6 kg of solvent-extracted linseed meal (24% of total DM intake) increased intake, digestibility, metabolizability and the total balance of energy and protein, but was less effective than soybean meal supplementation (Mehra et al., 2006).

Pigs

Linseed meal is a potential protein supplement for pigs, but its fibre content, the presence of antinutritional factors and its low lysine content tend to limit its utilisation in pig feeding. In young pigs (creep feed and started diets), only very low levels (up to 3%) are recommended (Bowland, 1990; Seerley, 1991; Chiba, 2001; Maddock et al., 2005). In one study, 15% of linseed meal in the diet had no effect on the performance of pigs between 6 and 11 weeks of age (Richter et al., 1997). It has been suggested that linseed meal could be used to best advantage at a level of up to 50% of the protein supplement (Seerley, 1991). Because it is deficient in lysine, linseed meal should be used in combination with lysine-rich protein sources. It has been reported that an inclusion rate of 5% reduced efficiency of feed and DE utilization in growing pigs (Bell et al., 1993). However, linseed meal could be used at levels between 5 and 10% in diets for growing-finishing pigs provided that the diet has been balanced for digestible amino acids (Li DeFa et al., 2000). The mucilage in linseed meal is indigestible by non-ruminant species, but it can absorb a large amount of water. Thus, linseed meal may have a laxative effect and be beneficial in preventing constipation in sows at parturition (Seerley, 1991). There appears to be a lack of published data on the use of linseed meal for gestating and lactating sows. It has been suggested that at least 10% could be included in sow diets, provided the diets are correctly balanced for lysine (Bowland, 1990). Because of the softening property of the oil, linseed cake is unsatisfactory as a main ingredient in pig feeds but solvent-extracted meal is unlikely to have any softening effect (Göhl, 1982).

Poultry

Though it can supply valuable linolenic acid to poultry diets, thus improve egg and meat quality, linseed meal is generally considered to have a poor nutritive value for poultry (Doreau et al., 1997). It is unpalatable, potentially toxic to poultry, and, while rich in protein, it is deficient in lysine. It contains mucilaginous gums that are deleterious to poultry as they cause sticky droppings, and reduce growth rate and digestibility (Alzueta et al., 2003; Kratzer et al., 1996). For those reasons, linseed meal should not be used in broiler diets at levels above 2.5-5%, and not above 10% in layers (Halle et al., 2013; Jeroch et al., 2008; El Boushy et al., 2000).

In growing pullets, expeller linseed meal satisfactorily replaced 50% of the protein supplied by soybean meal in diets (17-18% dietary level) where the diet was supplemented with methionine and pyridoxine, the latter to counteract the effects of linatine, a pyridoxine antagonist (see Potential constraints above) (Wylie et al., 1972). Treatments such as autoclaving and water extraction have been assessed to alleviate the deleterious effects of mucilages. Boiling in water was found efficient to reduce the mucilage content and birds fed boiled linseed meal at 50-70% of dietary protein maintained their growth rate (Madhusudhan et al., 1986).

In laying hens, early experiments recommended to limit linseed meal below 2.5% (Ewing, 1997). More recent trials in Germany have suggested that it is possible to include solvent-extracted linseed meal in layer diets at 10 or 15% of the diet in order to increase the linoleic acid content in egg yolks (Jeroch et al., 2008; Kirchgessner, 2004 cited by Jeroch et al., 2008). A 10% inclusion rate was shown to be a good trade-off between animal performance and egg quality (Halle et al., 2013).

Rabbits

Linseed meal is a traditional source of protein in rabbit feeding, and the most usual inclusion rate is 8-10%, both in diets for fattening rabbit and breeding does (Bruce et al., 1946; Aitken et al., 1962; Varenne et al., 1963; Scheelje et al., 1967). In more recent works, linseed meal was used safely at up to 28% in experimental diets (Krishna et al., 1990), or up to 45% in concentrates for Angora rabbits distributed together with Grewia optiva leaves used as forage (Singh et al., 1987). When formulating rabbit diets, the poor lysine content of linseed meal (which meets about 78% of the requirements of growing rabbits) and the limited concentration of sulphur-containing amino acids (95% of requirements) must be taken into account to prevent a fall in growth rate (Gippert, 1980; Lebas, 2004).

In balanced diets, the use of linseed meal is generally associated with an improvement of the digestive health of rabbits (Schlolaut et al., 1995). This improvement is probably due to the residual linseed oil, since this effect has also been observed with extruded linseeds (Colin et al., 2012) and linseed oil (Szendrö et al., 2012). Linseed oil is rich in alpha-linolenic acid, a fatty acid linked to improvements in the health of breeding and growing rabbits (Colin et al., 2012). Depending on the amount of residual oil, adding linseed meal to rabbit diets may increase the content of omega 3 fatty acids in the meat: +1% of linolenic acid in the diet induced an increase of 1.3% of linolenic acid in rabbit meat (Lebas, 2007).

Nile tilapia (Oreochromis niloticus)

In Nile tilapia fingerlings, a mixture of 4 oil meals, including 25% of linseed meal, replaced 75% of fish meal protein (55% of the diet) with no deleterious effects on fish growth (El-Saidy et al., 2003). Linseed meal included at 50% of the diet to replace 100% of fish meal in isonitrogenous and isocaloric diets reduced feed intake and growth (Gaber, 2006).

Rohu (Labeo rohita)

In rohu fingerlings, the apparent and true protein digestibility of linseed meal were 82 and 86% respectively, only slightly lower than for soybean meal (84 and 89%) (Hossain et al., 1997). Fermentation of linseed meal with Lactobacillus acidophilus had a positive effect on its nutritive value by reducing tannins and phytates. However, fermented linseed meal included at 25 to 75% replacement of fish meal protein in rohu fingerlings diets depressed performance and feed conversion ratio, unless the diet was supplemented with amino acids (lysine and methionine + cystine), in which case it could be used to replace 50% of fish meal protein (Mukhopadhyay et al., 2001).